This invention relates to machines for tying wire bindings around reinforcement bars as used in the construction of reinforced concrete.
WO 2007/042785 gives an example of a wire binding machine used for tying wire loops around intersections of steel reinforcement bars for constructing reinforced concrete structures. The design of machine shown in this document has been shown to produce tight and reliable ties in a practical and compact package. However as with any battery-powered tool, it would always be desirable to be able to reduce its power consumption even further in order to extend battery life or allow a smaller and therefore lighter battery to be used.
The Applicant has now appreciated that one area where a reduction in power consumption might be possible is in the motor used to feed the wire from the spool to the head and to withdraw it again to pull the loop tight prior to spinning.
When viewed from a first aspect the present invention provides a machine for tying a length of wire around one or more objects comprising a wire feed mechanism adapted to feed wire from a spool during a first phase; and to withdraw the wire during a second phase, said wire feed mechanism comprising a gripping mechanism including a pair of rollers urged together to grip the wire therebetween and drive it in the appropriate direction, said gripping mechanism being configured such that during said second phase, increasing tension in the wire automatically increases the gripping force on the wire.
Thus it will be seen by those skilled in the art that in accordance with the invention the grip on the wire increases with wire tension during the second, retraction phase. The invention involves a recognition by the Applicant that a much greater gripping force on the wire is required in the second phase, especially during the latter part thereof if the wire is to be pulled tightly around the reinforcement bars. It has been recognised accordingly that during the first phase there is a lower gripping force requirement as it is only necessary for the drive mechanism to overcome the friction encountered by the wire in being withdrawn from the spool and fed through the machine.
In previously proposed arrangements the grip on the wire was set at a constant high value to ensure sufficient tension could be applied to it during the second, retraction phase to ensure a good tie. However this meant the torque in the driving motor and so the current used by the drive mechanism was higher than it needed to be in the first phase. By employing an automatically increasing grip as the tension in the wire increases as result of wire is drawn tightly, the grip and so current drawn can be kept low during the first phase without compromising how tightly the loop can be drawn during the second phase.